The invention concerns a process for cold rolling strip composed, for example, of special steels and/or nonferrous metals in a multi-stand tandem rolling mill, in which the first stand(s) carry out relatively large reductions, and the last stand(s) determine the surface quality of the strip with relatively low reduction. The invention also concerns an installation for carrying out the process.
Strip cold-rolling processes have two basic objectives: first, with respect to the rolling stock, to achieve the greatest possible draft per pass with comparatively low demands with respect to the surface quality of the product and, second, with respect to the finished product, to achieve an optimum specific surface quality with comparatively low reduction. To achieve this twofold objective, previous rolling technology has always provided for the use of only one lubricant/coolant (emulsion or oil), and compromises had to be accepted in various rolling programs.
Rolling oil used as a cold-rolling lubricant has lubricating properties that are better than those of an emulsion by a factor of 1.3. Together with selected additives, this oil always produces a homogeneous film on the surface of the strip, which has the advantage of efficient separation of strip surface and deforming roll. Therefore, the resulting strip surface is extremely homogeneous and has a fine structure. A high surface brightness can be produced. Although rolling oil is relatively expensive, it is easy to handle, so that a more or less unlimited service life is achieved with low maintenance costs. However, rolling oil has the critical disadvantage that it is a very poor heat conduction medium and also has a low heat capacity. This results in elevated temperatures of the strip and roll during the rolling process, so that in unfavorable cases the work rolls reach maximum allowable temperatures, and the strip itself has temper colors. Due to the lower cooling effect, flatness control by zone cooling proceeds less effectively than with the use of media with a higher cooling effect.
Although the disadvantage of the poor cooling effect of rolling oil can be at least partially eliminated by the use of an emulsion as the rolling aid, the lubricating effect of an emulsion is determined mainly by the droplet size and by the saponification number. However, the lubrication is significantly less homogeneous, which is the reason that the emulsion oil that is used must contain special substances, if lubrication that is satisfactory to some degree is to be obtained. These two characteristics and difficulties result in a surface quality that is not as good. In addition, the filterability of an emulsion is limited due to the size of the oil droplets. A fine or extremely fine filtration cannot be performed, because it would cause phase separation, which would have negative consequences for the surface of the strip.
Weighing the specific characteristics of the different lubricants and coolants leads to the conclusion that, to produce high-grade cold-rolled strip made of special steel in tandem rolling mills, new methods must be sought to guarantee strip surfaces of the highest quality.
The document WO 98/51,423 describes a tandem rolling mill with two stands for the production of thin steel strip in gages as small as 0.18 mm. These stands achieve a reduction of usually 50-60%. To achieve further reduction, a final cold-rolling pass is carried out. However, this does not make it possible to go below a certain thickness limit, because that would lead to inferior and unsatisfactory product quality.
The rolling mill described in WO 98/51,423 has a first lubrication system and a second lubrication system, which can be alternately connected, depending on the given operating state. Different circulations using separate tanks are provided to prevent mixing of the lubricant fluids. Each tank is equipped with its own means for controlling and filling the liquid level and its own means for monitoring the composition of the lubricant. In accordance with one variant, the monitoring devices for the composition of the lubricant are accompanied by means for at least partially restoring the physical/chemical characteristics of the lubricants. Proportioning devices for substances such as lubricating oil, additives, inoculating agents, etc., are provided, which are suitable for restoring or reestablishing the lubricant characteristics. Stirring devices are used to maintain a uniform concentration of the lubricating fluid at all times to prevent deposits and contamination. The reference to “concentration of oil in the lubrication mixture” shows that, in this method, they were thinking of the use of emulsions with different concentrations. The alternative use of homogeneous monodisperse rolling oils is not the object of the document cited above.
The document 770 573 of the Union of Soviet Socialist Republics (USSR) describes a process for rolling and temper rolling on a two-stand rolling mill, which involves the shaping of the metal with a reduction in the first stand of 40-60% and in the second stand down to as little as 5% with lubrication and roll cooling. To increase the efficiency of the process,. to reduce the power consumption for the shaping and to obtain high-quality rolled strip with a flawless surface finish, the lubrication and cooling are carried out only in the first stand. An emulsion of high-viscosity mineral oils with a concentration of up to 1% is used as the lubricant, and an aqueous surfactant solution with a concentration of up to 0.001 moles/L is used as the coolant, and a wash solution is applied to the strip before and after the second stand.